High strand count plastic net

ABSTRACT

The present invention is directed to the manufacture of plastic net formed by extruding a plurality of spaced strands of plastic into a net structure. In accordance with the invention, adjacent strands in the net are joined in physical contact throughout all or a substantial portion of the length of the strand between the joints in the net structure. The strands are merged and integrally consolidated in the joints so that in effect the extruded structure is a solid sheet with or without perforations between the strands. Thereafter the strands are stretched preferably in two different directions to draw the strands apart and separate them throughout the length of the strands between the joints to form a high strand count oriented net produced for the first time by extrusion.

RELATED APPLICATIONS

The pending application is a divisional of Ser. No. 785,862, filed Apr.8, 1977, now U.S. Pat. No. 4,123,491, which was a continuation-in-partof Ser. No. 653,474, filed Jan. 29, 1976, now abandoned, and Ser. No.653,541, filed Jan. 29, 1976, now U.S. Pat. No. 4,152,479. The latterwas a continuation of Ser. No. 697,492, filed Jan. 12, 1968, nowabandoned.

BACKGROUND OF THE INVENTION

The continuous extrusion of plastic net started in about 1956 inaccordance with the process of the Mercer U.S. Pat. No. 2,919,467. Theprocess is carried out by means of two rotating die members. In essence,there is an outer rotating die member in the form of a flat horizontalcircular plate having an opening in the center thereof and an innerrotating die member in the form of a flat circular plate which is nestedin the opening of the outer die member. Sliding contact between the twodie members establishes a liquid seal. Each die member has a pluralityof spaced orifices in the form of open grooves in the contactingsurfaces between the two die members. Strands of plastic arecontinuously extruded in vertical direction in a circle through the opengroove orifices and the die members are preferably counter rotated sothat each time an open groove orifice in the outer rotating die memberis aligned with an open groove orifice in the inner die member anintegral joint is formed between adjacent strands. The resulting productis an extruded tube of plastic net which in general has diamond shapedopenings extending along the length of the tube. The Mercer process hasbeen and now is widely used and there are licensees using the process inall of the more important industrial countries in the world.

The United States and French licensees of the Mercer process eachindependently made a modification wherein the outer die is heldstationary while the inner die member is reciprocated in verticaldirection to separate the contacting surfaces between the two diemembers. In the preferred structure, there are no open groove orificesin the contacting surface of one of the die members. A plurality ofspaced plastic strands are continuously extruded vertically in a circlethrough the open groove spaced orifices in the stationary outer diemember. The inner die member is periodically separated from the outerdie member to separate the contacting surfaces and thereby extrude atransverse circular strand that forms an integral joint that connectsthe transverse circular strand with all of the spaced vertical strands.As a result a tube of plastic net is extruded which in general hassquare shaped openings extending along the length of the tube. Thiscontinuous extrusion of square mesh net is described in the Hureau U.S.Pat. No. 3,252,181 and in the Galt U.S. Pat. No. 3,384,692 and in theLemelson Reissue U.S. Pat. No. RE 28,600. Square mesh net is also beingextruded commercially in all of the more important industrial countriesthroughout the world.

There are also a number of other modified forms of the basic Mercerprocess in use. In one form plastic strands are continuously extrudedthrough a plurality of spaced orifice openings arranged in a straightline in a stationary die member. There are a plurality of spacedcooperating nozzles having orifices therein which are moved back andforth between two adjacent orifices in the stationary die member.Plastic strands are continuously extruded through the nozzle orificesand each time the nozzle orifice contacts an orifice in the stationarydie the two strands are welded together and as the nozzle moves away,the strands separate until the nozzle orifice contacts the next adjacentorifice in the stationary die member when these two strands are weldedtogether. As a result a plastic net is formed with a diamond typeopening along the length of the extruded net. The net may be extruded asa flat sheet or as a tube by using a circular stationary die member.This modified form of the basic Mercer process is described in the NalleU.S. Pat. No. 3,127,298. It is not necessary to use the Nalle nozzleorifices. For example, a plurality of spaced open groove orifices may bearranged in the contacting surfaces of each of two flat plate diemembers. At least one of the plates is adapted to slide back and forthso that any one groove orifice in the moving plate will make contactwith and move between two adjacent groove orifices in the stationaryplate while plastic strands are continuously extruded through theorifices. As a result, the strand from any one groove orifice in themoving plate continuously forms connecting links between two adjacentstrands extruded by the stationary plate to form a net structure. Thismodified form of apparatus is shown in FIG. 8 of the Mercer U.S. Pat.No. 2,919,467. Instead of sliding the moving plate, it may beperiodically displaced relative to the stationary plate to separate thetwo contacting surfaces. In this case, the contacting surface of themoving plate may be smooth without any groove orifices therein. When thesurface of the plates are in contact a first plurality of spaced strandswill be extruded from the stationary plate and when the contactingsurfaces are periodically separated a transverse strand will be extrudedto connect all of the first plurality of spaced strands into a netstructure with integral joints. This modification is shown in FIG. 21 ofthe Hureau U.S. Pat. No. 3,252,181.

In another modified form of apparatus, plastic is continuously extrudedthrough an open slit orifice which can be arranged in a circle orstraight line in a first stationary die member. A second cooperatingmoveable die member is employed to stop the flow of plastic at one ormore spaced areas along the fixed orifice opening in the stationary diemember. In the most simple form of structure, the second moveable diemember is in the form of a comb that slides back and forth across theorifice slit in the stationary die member. When the teeth of the combare retracted from the slit a continuous strand of plastic is extrudedand when the teeth of the comb interrupt the slit a plurality of spacedstrands are extruded transverse to the continuous strand of the slit.All of the strands are integrally joined together in the resulting netstructure. This modification is shown in FIG. 1 of the Hureau U.S. Pat.No. 3,252,181. The same principal may be employed in extruding a tube ofnet from an annular slit orifice as illustrated in FIG. 12 of the HureauU.S. Pat. No. 3,252,181. In the place of a comb, the moveable die membermay be in the form of a flat plate having an open slit therein in theform of a comb or any other desired configuration. The slit in themoving plate periodically interrupts the fixed slit opening in thestationary die member to form a net structure. This modification isshown in FIG. 10 of the Hureau U.S. Pat. No. 3,252,181.

The disclosures of the above-mentioned issued patent documents areincorporated by reference into the present specification.

In all of these processes for extruding plastic net, at least one set ofstrands is extruded through a plurality of spaced individual orifices.The second set of strands in the net structure may be extruded through asecond set of spaced individual orifices or the second set of strandsmay be extruded periodically through a continuous orifice slit arrangedin a straight line or in a circle. In all cases the two sets of strandsare extruded so that the individual strands intersect at an angle andform integral joints in the extruded plastic net. The resulting extrudedflat sheet or tube of plastic net is cooled to set the plastic in thestrands as for example in a water bath and the net is drawn away fromthe extrusion orifices by nip rolls or other suitable drawing means.When a tube of net is extruded, it is usually drawn down over acylindrical mandrel which may stretch the strands and enlarge theopenings in the net structure. Stretching the strands over the mandrelpartially orients the plastic but in practice the net is characterizedas being unoriented.

For many applications, it is desirable to further stretch the netstrands and more fully orient the plastic and this may be done byso-called rope orientation where as in the case of net with diamondshaped holes the tube of extruded net is heated and stretchedlongitudinally to further elongate and orient the strands. Stretchingthe tube causes it to collapse while the tube is stretched out in ropeform. Square mesh net is preferably oriented to advantage in a two stepprocess as described in my copending application Ser. No. 653,541. Here,a flat sheet of extruded net is heated and one set of strands is furtherstretched and oriented in one direction and in a second separate stepthe second set of strands are stretched to orient the strands in asecond direction. Some plastic net may be oriented at room temperaturebut as a practical matter the net is heated to speed up and facilitateorientation of the net.

In general extruded plastic net is classified according to the strandcount and weight. The strand count is the number of strands per linearinch in each set of strands. The strand count in a net with square ordiamond shaped holes may be readily determined by counting the number ofholes and fractions thereof per inch of net. The measurement is made ata right angle to one set of strands starting at the center line of aselected strand and the number of holes and fractions thereof in oneinch is recorded. A second measurement is made in the same way at aright angle to the second set of strands and recorded. Thus, if thereare 4.5 holes per inch of net for one set of strands and 5.5 holes forthe second set of strands in one inch of net structure, there will be4.5 strands per inch for the one set of strands and 5.5 strands per inchfor the second set of strands. For convenience this net is characterizedas a 4.5×5.5 or 5.5×4.5 strand count net. A 6×6 strand count net has 6strands (holes) per inch in each set of strands in each of twodirections. The expression 4×5 strand count net or 6×6 strand count netetc. as used in the specification and claims is intended to mean thenumber of strands per inch in each of the two sets of strands per squareinch of net. Weight of the net is usually specified in pounds per onethousand square feet of net.

Plastic net is also made from an extruded flat sheet of plastic byslitting, cutting, perforating or embossing the sheet in a predeterminedmanner to provide a multitude of small spaced openings of selectedgeometric configuration. In the embossing process, a multitude of smallnon-connecting depressions are formed with only a very thin membrane ofplastic at the bottom of the depressions which may extend all or onlypart way across the bottom of each depression. The net is formed bystretching the sheet usually in two different directions as for examplein a longitudinal direction and in a transverse direction at a rightangle thereto. The stretching in two directions may be carried outsimultaneously or in two separate sequential steps. Stretching the sheetcauses the embossed membranes to split and fibrillate and it otherwiseenlarges and expands the openings with corresponding elongation andattenuation of the full thickness portions of the sheet. Plastic net isproduced from an extruded solid sheet of plastic by a number ofcompanies in the United States and the processes are described inBritish Pat. No. 1,075,487 (U.S. Pat. No. 3,441,638) issued in 1967 toT. J. Smith and Nephew Limited which is incorporated by referenceherein. Also see British Pat. No. 982,036 issued Feb. 3, 1965 to FMCCorporation and U.S. Pat. No. 3,881,381 issued May 6, 1975 to Johnsonand Johnson and U.S. Pat. No. 3,666,609 issued May 30, 1972 to Johnsonand Johnson.

THE INVENTION

Net formed by extruding strands of plastic up until now has not beenfully competitive with the net formed from an extruded solid sheet ofplastic. In producing net by extruding strands, the strands are extrudedas sharply defined spaced individual strands in order to maintain theintegrity of individual strands in the net structure. As a result, thefinal unoriented extruded net up until now has had a maximum strandcount of about 18×18 which is equivalent to a balanced-net hole densityat the die lips of about 500 or less openings per square inch. Whenfully oriented this commercial net has a strand count of about 6×6.There is no particular limitation in the strand count of net formed froman extruded solid sheet of plastic and the fully oriented commercialproduct may have a strand count of 9×9 or more. In some commercialapplications, it is important to have a high strand count net beyondthat heretofore produced in net formed by extruding individual strands.

It has now been discovered that it is not necessary to extrude sharplydefined individual spaced strands in order to maintain integrity of thestrands in the oriented net structure and an entirely satisfactoryoriented net of high strand count has been produced. In accordance withthe present invention, adjacent strands are extruded and compacted tomake physical contact over all or a substantial portion of the length ofthe strand between the joints of the two sets of strands. The strandsare merged and integrally consolidated in the joints so that in effectthe extruded structure is a sheet with or without perforations betweenthe joints. If the net is extruded as a tube, the tube is preferablyslit in a longitudinal direction and the resulting sheet is heated andstretched in at least one direction and preferably in two directions.Best results are achieved by stretching the sheet in a first directionalong the length of one set of regenerated strands and by stretching thesheet in a second separate step in a second direction along the lengthof the second set of regenerated strands.

It was quite unexpected to find that when the sheet is stretched,strands are regenerated as by attenuation of the joints and separatedinto well defined spaced individual strands throughout the length of thestrands between the joints of the two sets of strands. Even thoughadjacent strands are joined together at the time of extrusion or justthereafter, the integrity of the strands is apparently maintained in theextruded structure sufficiently well to be regenerated when drawn apartduring the stretching to form well defined individual strands in thefully oriented net structure. While the preferred method of stretchingthe extruded sheet involves a two step process, it will be understoodthat the extruded structure may be stretched in two directionssimultaneously or only in a single direction or the extruded structuremay be stretched while in the form of a tube.

The high strand count oriented net produced by extruding strands hasbetter tensile strength and resistance to tear than comparable netproduced from an extruded sheet of plastic and the extruded strands arewell defined as compared to the strands produced from a sheet of plasticwhich tend to be fibrillated.

THE DRAWINGS

Further details of the invention and one preferred type of extrusionapparatus may be readily understood by reference to the followingdescription and drawings in which:

FIG. 1 is a sketch of one-half of one preferred extrusion die head forcarrying out the present invention partly in section to betterillustrate the parts;

FIG. 2 is a photograph of an extruded structure produced in accordancewith the present invention;

FIG. 3 is a photograph of the strands and joint of the oriented netproduced from the extruded structure of FIG. 2 by stretching to open itup into a net;

FIG. 4 is a photograph of an oriented net produced from an embossedsheet of plastic;

FIG. 5 is a diagrammatic illustration in side elevation of one preferredform of orienting apparatus;

FIG. 6 is a diagrammatic illustration in top plan view of the apparatusof FIG. 5; and

FIG. 7 illustrates the joint in an extruded oriented plastic net whichhas been webbed.

Turning now to FIG. 1, the extrusion die head comprises an outer annularstationary die member 24 having a circular interior opening 26 therein.A plurality of spaced individual open groove orifices 28 are positionedin die member 24. A reciprocating piston 30 preferably without anyorifices therein is nested in the opening 27 and as shown in FIG. 1 thesmooth surface at the nose of piston 30 forms a seal with the lands ofthe grooved orifices 28 as at 32 when the piston is in its lowermostposition as shown. The orifice grooves 28 remain open at all times tocontinuously extrude a spaced plurality of machine direction strands 34in a circle. The plastic is continuously supplied to orifices 28 bymeans of an annular feed channel 36. Piston 30 is also in a sliding andsealing engagement with the inner wall of the stationary annular mandrel38 and the outer annular surface of mandrel 38 forms the inner annularwall of the plastic feed channel 36. Piston 30 is attached in fixedposition to a drive rod 40 which causes the piston to reciprocate up anddown and into and out of engagement with the lands of stationary die 24.Each time the piston is lifted out of engagement with the stationary diemember 24, an annular slit orifice defined by the distance between thelands of orifices 28 and the nose of the piston 30 periodically extrudesan annular transverse strand 42 of plastic to form a second set oftransverse strands 42 which are connected to the machine directionlongitudinal set of strands 34 at each place where the strandsintersect. The resulting tube of net is drawn downwardly away from theextrusion die head over a cylindrical mandrel and through a water bathpreferably by a pair of nip rolls (not shown). After the plastic strandshave set the tube of net is preferably slit longitudinally and openedinto a flat sheet which is accumulated on a wind up roll (not shown).

In accordance with the present invention, the strands extruded with thedie head of FIG. 1 are compacted and merged to form integrallyconsolidated joints so that there is physical contact between adjacentstrands throughout all or a substantial portion of the length of theextruded strand between joints. This compacting to merge the strands inintegrally consolidated joints occurs at the lips of the die or adjacentthereto and in the high strand count net of the present invention, theextruded porous structure as measured at the die lips will have abalanced-net hole density of at least 1200 holes per square inch of netstructure.

The term balanced-net hole density of 1200, 1800 etc. as used in thespecification and claims is defined to be the specified number of holesin a balanced structure of a square mesh net having the same number ofstrands in each set of strands in one square inch of the extrudedstructure at the die lips without any expansion of the structure such asmay be caused by drawing it away from the die. An extruded structurehaving a balanced-net hole density of 1200 has a strand count of about34.6×34.6 at the die lips before any expansion. Unbalanced extrudedstructures having a different number of strands in each set of strandsin each square inch of structure equivalent to a balanced-net holedensity of 1200 is obvious to those skilled in the art. For example, asquare mesh net (rectangular openings) having a strand count of 30×40 or20×60 is equivalent to a balanced-net hole density of 1200 at the dielips before any expansion.

In one example of the manufacture of plastic net in accordance with thepresent invention, the annular stationary die of FIG. 1 had a diameterof 7.874 inches as measured at the lips of the die. This is equivalentto a circumference at the die lips of 24.737 inches and 1080 open grooveorifices (28) were equally spaced around the circumference of the diemember 24. The width of each groove orifice was 0.011 inch with a depthof 0.011 inch. The extruded porous structure had a balanced-net holedensity at the die lips before any expansion of 1906 holes per squareinch of structure with a strand count of about 43.7×43.7. Approximately43.7 strands per inch of die circumference were extruded in machinedirection by the open groove orifices 28 and approximately 43.7transverse strands per inch of extruded structure were extruded throughthe slit annular orifice opening between the lands of orifices 28 andthe nose of the mandrel when separated from the lands of orifices 28.

Polypropylene was used in extruding the tubular net-like structure,which was drawn away from the die lips over a mandrel in the water bath.The mandrel expanded the porous structure to the final extruded net holedensity of about 800 holes per square inch. The tube was thereafter slitand the porous sheet taken up on a wind up roll.

FIG. 2 is a photograph of the extruded polypropylene porous structure ofthis example. The photograph of FIG. 2 is a sample of the sheet takenfrom the wind up roll enlarged fifty times. As illustrated in thephotograph the extruded polypropylene structure is a porous sheet withadjacent net strands merged and integrally consolidated in the joints.

The polypropylene structure of FIG. 2 was thereafter taken from the windup roll and oriented to produce an unbalanced net having a hole densityequivalent to a balanced-oriented net hole density of 82 and a strandcount of 9.7×8.5. This oriented net is shown in FIG. 3 which is aphotograph of a sample of the oriented net enlarged fifty times. As bestshown by a comparison of the photographs of FIGS. 2 and 3, theindividual strands were regenerated from the porous extruded sheet anddrawn out and separated from the joints during orientation and thestrands although pulled apart were very well defined. The stretchingduring orientation of the extruded porous structure of the presentinvention expands and elongates the structure which results in a highstrand count oriented net having a balanced-oriented net hole density ofat least 49 and preferably the balanced-oriented net hole density willbe 64 or greater.

The term balanced-oriented net hole density of 49, 64 etc. as used inthe specification and claims is defined to be the specified number ofholes in a balanced oriented net structure of a square mesh oriented nethaving the same number of strands in each set of strands in one squareinch of the net in the final commercial product after orientation.Unbalanced oriented nets having a different number of strands in eachset in a square inch of net will have a hole density equivalent to thatspecified for the balanced oriented net.

FIG. 4 is a photograph of a sample of the commercial net manufactured bythe Smith and Nephews process produced by embossing an extruded solidsheet which is oriented to form the net as described in the Smith andNephews British Pat. No. 1,075,487. In this photograph the sample isenlarged fifty times. As illustrated in the photograph, the stretchingof the embossed sheet to open it up into a net structure of connectingstrands causes extreme fibrillation and there are no well definedstrands such as those produced in the oriented net of the presentinvention. In both cases, the plastic material is attenuated bystretching the structure to open it up into a net but in accordance withthe present invention well defined strands are produced in theorientation process as compared to the fibrillated strands produced fromthe embossed sheet. The embossed sheet type of fibrillation shown inFIG. 4 also occurs when oriented net is produced from an extruded sheetof plastic which has been slit, cut or perforated with or withoutembossing. Stretching the extruded porous structure of the presentinvention to expand it and produce the oriented net may be carried outin conventional known manner. In general the plastic is heated and thenstretched. If desired the structure may be stretched while in the formof a tube but it is of advantage to stretch and orient a sheet of theporous structure in accordance with the teaching of my copendingapplication Ser. No. 653,541. As described in my copending applicationthe extruded structure of the present invention is stretched in a firstdirection as along its length to produce longitudinal strands and in asecond subsequent step it is stretched in a second transverse directionto produce the transverse strands. Stretching in this manner tends toincrease the heat stability of the resulting net and for strength thestretching is controlled to avoid visible webbing at the joints of thestrands.

The apparatus for stretching the porous structure of the presentinvention in accordance with the teaching of my copending application isillustrated in FIGS. 5 and 6.

Having reference now to the drawings, FIG. 5 shows schematically theorienting device as comprising a roll holder 110 for holding the infeedroll 122 of the extruded net 120. Reading from right to left in FIG. 5,the successive elements of the device, after the roll holder 110, are alongitudinal stretching mechanism 112, an oven and transverse stretchingmechanism 114, a cooling mechanism 116, and a takeup mechanism 118. Theroll 122 of net material is removably journalled at 124 in the frame 126which also supports various idler rolls 128. The net 120 is fed from theroll 122 around the idler rolls 128 and into the longitudinal stretchingmechanism 112 which comprises a frame supporting various idler rolls 132and three large heated rolls 134, 136 and 138. Disposed between the twolarge heated rolls 136 and 138 is a set of four rolls comprising twoidler rolls 142 and 144, and two longitudinal stretching rolls 146 and148.

After leaving the longitudinal stretching section 112 and the variousrolls supported therein, the net is transported to and through atransverse stretching mechanism 114 comprising an oven 140 and twospaced, horizontally-disposed, endless chain members 150 and 152 (seeFIG. 6). The endless chain members 150 and 152 have suitable grippers(not shown) thereon for gripping the edges 154 and 156, respectively, ofthe net 120 for transporting the same through the oven 140 and forpulling the net transversely as hereinafter described.

After issuing from the transverse stretching mechanism 114, the net iscarried by the endless chains 150, 152 over a cooling section 116comprised of one or a plurality of blowers 158 which blow cooling airthrough the net 120. After leaving the cooling section 116, the net 120passes over suitable idler rolls as illustrated, for example, by rolls162 and 164, and is taken up on the roll 160 of the takeup section 118.The roll 160 is formed on a shaft 166 which is suitably driven by amechanism not shown to take up the net 120. Both edges of the sheet ofnet are preferably trimmed and removed before winding.

It is to be understood that various idler rolls are used and that thoseshown are for illustrative purposes only since requirements of space andequipment may require different paths of travel for the net 120.Generally, the rolls 134, 136, 138, 146 and 148 as well as shaft 166 aredriven at suitable speeds as hereinafter described to convey the net 120through the device. The endless chains 150 and 152 are also suitablydriven in the direction shown by the arrows 168 and 172, respectively.The other rolls shown in the device are generally idler rolls and neednot be driven except to remove tension or friction from the net and, ifdriven, are driven at the speed of the net at the point where such netcontacts any such given roll. The linear speed of the net 120 throughthe device is, however, not constant as will now be described.

The driven rolls 134, 136, and 148 are driven at such speeds of rotationthat their peripheral speeds are about the same. The rolls 138 and 146,however, are driven at a peripheral speed greater than that of the rolls134, 136 and 148. Generally, the peripheral speed of the rolls 134, 136and 148 is the same as the linear speed of the net 120 as it leaves theroll 122 and passes over the various idler rolls to and through therolls 134, 136 and 148.

The rolls 138 and 146 are driven at the same peripheral speed, whichspeed is that of the net 120 as it passes over such rolls and throughthe remainder of the apparatus including the transverse stretchingmechanism 140, the cooling mechanism 116, and the takeup mechanism 118.

It will be realized that since the rolls 146 and 138 operate at a higherperipheral speed than the rolls 134, 136 and 148, the net 120, ifproperly heated, will stretch longitudinally in the space between therolls 146 and 148 due to the difference in speed of these two rolls. Therolls 142 and 144 may also be driven, if desired, with the rolls 142driven at the same peripheral speed as rolls 138 and 146 and roll 144being driven at the same peripheral speed as rolls 134, 136 and 148.

The rolls 134 and 136 are heated to such temperature as may be requiredby the circumstances of the particular orientation mechanism and theparticular resin composition of the net 120. For certain polypropyleneresins, it has been found that a suitable temperature for these rolls isin the range of from 200° F. to 300° F. The means for heating theserolls may include the use of water, oil, or other liquid, heated andpumped through such rolls, such means not being shown since many suchmechanisms are known.

The roll 138 is likewise heated but to a lesser temperature than therolls 134 and 136 to set the resin of the net 120 in order to permit itshandling and passage from the roll 138 over the area generally indicatedby the reference character 170 and into the transverse stretchingmechanism 114 where the net 120 is gripped by the endless chains 150 and152. It will be realized that in the area 170, the net 120 issubstantially unsupported and as such, must have some self-supportingstrength. Accordingly, the roll 138 is not as hot as the rolls 134 and136 in order that the net 120 may be slightly cooled to set the resin,thus permitting transfer across the space 170.

As indicated above, the net 120 is extended in length and thusstretch-oriented in its passage through the longitudinal stretchingmechanism 112, such stretching taking place between rolls 146 and 148 byvirtue of the different peripheral speeds of the rolls 146 and 148 andthe heated condition of the net at the time it passes over and betweenthese rolls. This is shown diagrammatically in FIG. 6 in which the net120 is shown as consisting of transverse strands 180 and longitudinalstrands 190. As best shown in the area between the infeed mechanism 110and the longitudinal stretching mechanism 112 in FIG. 6, it will berealized that the transverse strands 180 and the longitudinal strands190 define open mesh areas in the net 120 which are substantially squarein the embodiment shown. After having passed through the longitudinalstretching mechanism 112 the longitudinal strands 190 have beenstretched substantially and may be stretched two or more times theiroriginal length but the transverse strands 180 have remained at theiroriginal length as best shown in the area 170 in FIG. 6.

The transverse stretching mechanism 114 includes the oven 140 containinggenerally three sections as indicated by the reference characters 174,176 and 178. These sections of the oven 140 are heated by any suitablemeans including, for example, a hot gas blowing mechanism, such asindicated at 182. Generally, the section 174 is a preheating section,section 176 is a transverse stretching section and section 178 is aheat-setting section which may be kept at a higher temperature. For atypical polypropylene resin, the various sections 174, 176 and 178 arekept at a temperature in the range of from 250° F. to 325° F. Adjustmentof the temperature in the three sections is within the skill of the art.

Upon entering the transverse stretching mechanism 114, the net 120 isgripped on its edges 154 and 156 by suitable grippers or pins (notshown) on the endless chains 150 and 152, respectively. The endlesschains 150, 152 are arranged in a suitable track (not shown) whichguides such chains 150, 152 initially along two parallel straight pathsin the section 174 of the oven and then along diverging paths in thesection 176 of the oven as shown in FIG. 6, and then again alongparallel straight paths in section 178 of the oven. The paths of thechains 150, 152 in section 178 are more widely spaced than in section174.

The endless chains 150, 152 carry the net 120 through and out of theoven 140 and then over the cooling fans 158 which blow room temperatureair through the net to cool the same to or close to room temperature. Ator near the point where the chains 150 and 152 are trained around thesprockets 184 and 186, respectively, the net 120 is released from thegrippers of the chains 150 and 152 and is then taken up by the roller160 driven by the driving shaft 166.

By reference to FIG. 6 of the drawing, it will be seen that during thepassage of the net 120 through the section 176 of the oven, thetransverse strands 180 are stretched in length since the chains 150 and152 are diverging during this point of their travel. In the embodiment,the strands 180 are shown stretched to approximately three times theiroriginal length. At the same time, the longitudinal strands 190 are notincreased in length but retain the length given to them during travelthrough the longitudinal stretching device 112. This is evident fromFIG. 6 when one compares the net 120 as shown in the area 170 to the netas shown after having passed through the section 176 of the oven. At thepoint 170, the transverse and longitudinal strands 180 and 190,respectively, define relatively elongated rectangles elongated in thedirection of travel of the net. As shown after having issued from thesection 176 of the oven, however, the strands 180 and 190 are shown asdefining relatively large square openings in the net 120 of the sameshape but of different dimension as compared with the original net 120as it issued from the roll 122.

It is not necessary, of course, that the net 120 start out with squareopenings which are then transformed to elongated rectangles and thenfinally to larger square openings, such having been shown merely forclarity of illustration in the drawings. Obviously, as shown, thepassage of the net through the device is such that there occurs pullingand stretching of the net strands first in one rectilinear direction inthe longitudinal stretching mechanism 112 and secondly in a rightangular transverse direction in the transverse stretching mechanism 114.

In general the plastic structure to be stretched is heated at least to atemperature at which the molecules of the particular polymer may beoriented by stretching. One set of strands only is then stretched whileheated. After completion of this orientation-stretching, the structuremay or may not be cooled slightly depending on the design of theequipment employed. If cooled, the structure is again heated at least toa temperature suitable to permit the orientation of the molecules of theparticular polymer. While so heated or when so heated, the second set ofstrands only is stretched to orient the molecules. Finally the net iscooled.

The particular temperatures employed will depend upon a number ofvariables and must be determined empirically for each set ofcircumstances. It is generally considered that orientation of themolecules can take place at a second transition point, however, forpractical commercial operation the required temperature may beconsiderably higher. The second transition point for polypropylene, forexample, is approximately room temperature, and orientation can becarried out very slowly at this temperature. But as a practical matter,quantity commercial operation is carried out at temperatures generallyin the range of 200° F. to 325° F.

While the above description is directed to orienting conventional net,the extruded porous sheet of the present invention is preferablyoriented to draw out and separate the strands by using the two stepsequential process as hereinabove described. In general, for welldefined strands and maximum strength in the oriented structure of thepresent invention, the stretching is preferably carried out so as toavoid substantial visual webbing at the intersection of the strand andthe joint between the strands. Visual webbing is avoided by adjustingthe rate at which the structure is stretched to elongate it and byadjusting the temperatures at which the structure is stretched in thelongitudinal and transverse directions. The rate of stretching andtemperature for avoiding visual webbing will be different for differentplastic materials and for different strand thickness and strand count.

In general in order to avoid webbing, an oriented set of strands shouldnot be subjected to appreciably higher temperatures during the remainingsteps in the process than that temperature at which the particular setof strands was oriented. When a strand is stretched to orient it,stresses are set up in the elongated strand while the plastic in thejoint remains substantially unoriented or the orientation of the jointis less than that of the strand between adjacent joints. If the orientedstrand while held under restraint in the orienting apparatus issubjected to appreciably higher temperatures than the temperature atwhich the strand was oriented, the strand will tend to shrink in lengthand in so doing will form visible webbing at the intersection of thestrand and the joint. For example, using the apparatus of FIG. 5, thehigh strand count net of FIG. 2 was oriented by maintaining thetemperature of the heated rolls in the longitudinal stretching section112 at about 270° F., and in the oven 114 which encloses the transversestretching section at about 290° F. The resulting oriented net of FIG. 3does not have any visible webbing at the joints. FIG. 7 illustrates thejoint of a plastic net having visual webbing at the joint which tends toweaken the joint in the net. Comparing the joint 200 of FIG. 7 with thejoint of FIG. 3, it will be seen that the strands 202 and 204 havedecreased in length due to employing appreciably higher temperatures(such as about 350° F.) in subsequent process steps from thattemperature at which the strands were oriented and as a result plasticmaterial is pulled from the substantially unoriented joint to form webs206 and 208. Conditions of operation for specific equipment under whichthe orientation is carried out are determined empirically to avoidvisible webbing of the joints. It is obvious that webbing at the jointscan be avoided by allowing the net structure to shrink without restraintin order to compensate for shrinkage caused by release of stresses inthe strands.

In certain commercial applications, the oriented net may be subject totemperatures approaching those at which the net was oriented. As aresult, the net will shrink, unless it is heat set during manufacture.In accordance with the present invention, the oriented net may be heatset by subjecting the net to temperature higher than that employed inorientation while controlling net shrinkage caused by release ofstresses in the strands at the higher temperature. For example, in thecase of the net of FIGS. 2 and 3, the net after orientation was heat setat a temperature of 340° F. and the net was held under partial restraintto limit shrinkage of the transverse strands to less than about 10% oftheir oriented length. This relieves the stresses in the orientedstrands so that when the net is later subjected to temperaturesapproaching those of orientation in commercial application, theshrinkage of the net is reduced to acceptable commercial levels. Whileit is preferred to allow the net to shrink less than about 10% duringheat setting, this is not necessary and the net may be held underrestraint to preserve the dimensions of the oriented net.

While polypropylene is specifically mentioned in the description, itwill be apparent that any plastic material which can be readily orientedmay be utilized in carrying out the present invention.

In order to extrude a tubular porous sheet having a balanced-net holedensity of 1200, the annular stationary die member 24 of FIG. 1 isprovided with about 34.6 evenly spaced open groove orifices 28 per inchof circumference of the die member 24. For a balanced-net hole densityof 1324, the annular stationary die is provided with about 36.4 evenlyspaced open groove orifices per inch of circumference. Use of about 41.2evenly spaced open groove orifices per inch of circumference of diemember 24 will provided an extruded structure with a balanced-net holedensity of 1700 and about 58.2 evenly spaced open groove orifices perinch of circumference of die member 24 will provide an extrudedstructure of balanced-net hole density of 3388. Since by definition abalanced-net hole density in the extruded structure at the lips of thedie orifices will have the same number of strands in each direction thepiston 30 of the die of FIG. 1 is reciprocated at that cyclic speedrequired to provide the same number of transverse strands per inch oflength of extruded structure as the machine direction strands extrudedby the spaced open groove orifice 28. For example, for a balanced-nethole density of 3388, the piston is reciprocated at that cyclic ratewhich is required to extrude about 58.2 transverse strands per inch oflength of extruded net structure when there are about 58.2 evenly spacedopen groove orifices per inch of circumference of die member 24. In thecase of unbalanced extruded net-like structures, the cyclic rate ofreciprocation of piston 30 may be such that there will be more or lesstransverse strands in the extruded structure per inch of length ofextruded structure than are present in one inch of circumference of theextruded tubular structure. This same relationship between balanced andunbalanced extruded net-like structures applies to the various otherform of commercial extruders described hereinabove which are used forproducing plastic net by the extrusion of strands to form the structure.For example, in the Mercer apparatus of U.S. Pat. No. 2,919,467 whenthere are an equal number of orifices in each of the rotating diemembers, a balanced net structure is produced. If one die member hasmore or less orifices than the other die member an unbalanced netstructure will be produced.

Those skilled in the art will experience no difficulty in extruding thehigh strand count of the present invention. But, it is well to note thatthe particular selected plastic will exhibit die swell as it leaves theorifice of the die. The plastic will swell as it leaves the orifice sothat the resulting strand may have a greater diameter than the diameterof the orifice. Plastic with extremely high die swell will tend toproduce an extruded structure with greater physical contact between thestrands and smaller holes than that produced from the same die by aplastic with extremely low die swell. The rate at which the extrudedstructure is drawn away from the die lips will have an effect on dieswell and as the rate of draw increases the die swell will decrease. Itis of advantage to utilize a plastic which will exhibit die swell duringextrusion since die swell is correlated to molecular weight distributionand the higher the die swell the broader the distribution. Plastics withbroad molecular weight distribution will generally improve extrusionefficiency and are less difficult to control.

The swell of the extruded plastic strands as they exit from the lip ofthe spaced individual die orifices is an extremely complex phenomenawhich involves a number of interrelated parameters. However, the dieswell may be increased in an empirical manner by adjusting the extrusionconditions or the configuration of the spaced orifices employed forextrusion.

Temperature of the plastic material at the lips of the spaced dieorifices influences the die swell and a decrease in temperature willincrease the die swell of the resin strands.

The pressure at which the resin strands are extruded through the spacedindividual die orifices is another factor that influences the die swellof the strands. Increasing the pressure will increase the die swell.

The rate of extrusion of resin effects the die swell of the strands fromthe spaced individual die orifices. An increase in rate of extrusionwill increase the die swell.

Changing any one or more of the above parameters of temperature,pressure, rate of extrusion, drawdown, configuration and number oforifices in the die member may be employed for changing the die swell ofthe extruded strands.

The die swell of a particular selected resin is described as the ratioof:

    (D.sub.o /D)×100%=% Die Swell

D_(o) is the average diameter of the extruded strand and D is thediameter of the selected orifice. A tested resin strand with a die swellof 100% does not expand at all during extrusion through the orifice. Atested resin strand with a die swell of 200% has expanded to twice thediameter of the test orifice. Any conventional test apparatus may beemployed for testing the die swell of the selected plastic. For example,a cylindrical tube equipped with heating elements and fitted with anorifice opening at the bottom thereof and with pressure means forforcing the heated flowable plastic through the orifice may be employed.If the orifice and temperature and pressure of the test is closelyrelated to the orifice and temperature and pressure and rate ofextrusion employed in the commercial extrusion apparatus, the tested dieswell of the plastic will be closely related to the die swell in thecommercial extrusion. Die swell of the plastic strand extruded by thetest apparatus will not in general be the same as the die swell of thestrands extruded through the spaced orifices in commercial apparatus.But, a correlation between the tested die swell and any given commercialapparatus may be empirically established after a number of tests. Dieswell is determined in the test apparatus by extruding a strand ofselected plastic and by measuring the diameter of the strand extruded bythe test apparatus. In the extrusion of polypropylene net-likestructures in the apparatus of FIG. 1 in accordance with the presentinvention, the die swell of the plastic extruded from the lips of thedie was at least about 125% for best results and in some cases the dieswell was 150% and more.

It will be understood that the description and claims are intended tocover all changes and modifications of the preferred embodiment of theinvention herein chosen for the purpose of illustration.

What is claimed is:
 1. An extruded porous tubular structure comprising afirst plurality of extruded strands that intersect at an angle and arejoined to a second plurality of extruded strands to form a tubularstructure, the strands in said porous structure being compacted toprovide a net hole density equivalent to a balanced-net hole density ofat least
 1200. 2. An extruded porous sheet comprising a first pluralityof extruded strands that intersect at an angle and are joined to asecond plurality of extruded strands, the strands in said porous sheetbeing compacted to provide a net hole density equivalent to abalanced-net hole density of at least
 1200. 3. An extruded orientedsheet of plastic net comprising a first plurality of extruded strandsthat intersect at an angle and are joined to a second plurality ofextruded strands having a strand count in the sheet of at least about7×7 and a net hole density equivalent to a balanced-oriented net holedensity of at least about
 49. 4. An extruded structure comprising afirst plurality of extruded strands that intersect at an angle and arejoined to a second plurality of extruded strands, the strands in saidextruded structure being compacted to form a non-porous structure havinga net hole density equivalent to a balance net hole density of at least1200.
 5. An extruded plastic structure comprising a first plurality ofextruded strands that intersect at an angle and are joined to a secondplurality of extruded strands, the strands in said extruded structurebeing compacted to provide a net hole density equivalent to abalanced-net hole density of at least
 1200. 6. An extruded orientedsheet of plastic net comprising a first plurality of extruded strandsthat intersect at an angle and are joined to a second plurality ofextruded strands, said sheet of net having a net hole density equivalentto a balanced-oriented net hole density of at least
 49. 7. A high strandcount extruded oriented plastic net comprising a first plurality ofplastic strands which intersect and are joined to a second plurality ofplastic strands at an angle to form a high strand count net having a nethole density equivalent to a balanced-oriented net hole density of atleast 49 in which the strands have been heat set to relieve stresses inthe net strands produced by orientation.
 8. A high strand count extrudedplastic net comprising a first plurality of strands which intersect andare joined to a second plurality of plastic strands at an angle to forma high strand count net having a hole density equivalent to abalanced-net hole density of at least about 49 in which the strands ineach set of strands have been oriented by stretching along the length ofthe strands without formation of visible webbing at the joints in thenet structure.